Optical switching, multiplexing and demultiplexing, as well as the ability to add or drop signals at a specific wavelength, are critical components of a Wavelength Division Multiplexed (WDM) optical network. Such components are particularly necessary in an optical ring network, such as the networks described by A. F. Eliefaire in "Multiwavelength Survivable Ring Network Architecture" published in Globecom '89, Dallas, Tex., November 1989, and by A. F. Eliefaire et al in "Fiber-Amplifier Cascades With Gain Equalization in Multiwavelength Unidirectional Inter-Office Ring Networks," published in IEEE Photonics Tech. Letters, Vol. 5, No. 9, September, 1993. It should be noted that channels within a WDM signal may be distinguished by either their optical wavelength or optical frequency, and the terms frequency and wavelength will be interchangeably referred to for this purpose.
In particular, tunable add/drop optical filters are needed in WDM communication systems for selectively adding or dropping channels from WDM signals or needed in WDM cross-connects. Other known circuits have in the past attempted to provide this capability.
The first is an opto-acoustical filter, an example of which is described in an article by Kwok-Wai Cheung entitled "Acoustooptic Tunable Filters in Narrowband WDM Networks: System Issues and Network Applications" published in the IEEE Journal on Selected Areas in Communications, Vol. 8., No. 6, August 1990. Such filters are useful because of their broad, continuous electronic wavelength tunability and narrow filter bandwidth. Their multi-wavelength filtering capability also adds a dimension of flexibility to network design that heretofore could not be achieved with similar optical filters.
Another filter known to provide this add/drop capability is the conventional wavelength-space optical switch shown in FIG. 1, which will be described in detail hereinafter. Briefly, this device consists of two IXN demultiplexers which are connected to two 1.times.N multiplexers via a plurality of 2.times.2 optical switches. A desired frequency component may be added, and a signal of the same frequency may be dropped by proper connection of the 2.times.2 optical switches.
Accordingly, although tunable add/drop filters are known, all known circuits have inherent limitations which detract from the performance of a WDM optical network.
It is, therefore, an object of this invention to overcome such limitations and provide an improved add/drop optical filter with arbitrary channel arrangements by making use of the unique transmission properties of the Wavelength Grating Router (WGR).